The smell might be from not washing himself enough or just from his natural sweat, I'm cut, but I do know alot of guys who are not cut and they say that being clean is very important when you are not cut, the foreskin covers the head of the penis and sometimes this caues guys to sweat and sometimes even some urine stays around the head and dries up there cauesing a bad odor after a while, just be careful when you talk to him about this he might not even know this is a problem ok.
Good Luck
Steve

There is only one reason for a penis to smell fishy.
It is semen, or pre-come, breaking down. It's possible that he is leaking pre-come, just from the thought of being with you. Mention that the smell bothers you, and ask him to wash it. It won't be a problem. No guy wants to think that he may be offending a potential lover.

A fishy smell under the foreskin is normal in uncut guys, but it shouldn't be overpowering or offensive - if it is, it could be a sign he isn't pulling back the foreskin and washing under there everyday in the shower like he should, and if its really strong fishy smell, it could be a sign of a yeast or fungal infection under the foreskin.

The smell is from the natural sweating and oil secretion from the skin, trapped between two layers of skin that are always on top of each other. Often excess drops of urine and precum and semen get trapped under the foreskin too, and mix with the sweat and oils to further the odor and create smegma, along with dead skin cells that constantly come off the head and inner foreskin.

It is not the semen itself, but the sugars breaking down under a foreskin, or inside a vagina, that makes it smell fishy.
If you have gone down on a woman, you have likely smelled smegma, at least mildly.